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PHILADELPHIA (CBS) – Many stores ask customers for their ZIP code at check-out, a practice that is coming under attack from two directions.

Giving a merchant your ZIP code at check-out may not seem like a big deal, but courts in both California and Massachusetts have ruled that it is, in fact, big, and now illegal in those states.

Lance Haver, director Philadelphia’s Office of Consumer Advocate, says merchants collect this information both to sharpen their own sales pitch to you, and often to sell aggregated data to other marketers.

“I’m particularly concerned when people give up private information at drug stores. I don’t know if people realize that that information can then be sold and, as we enter a new world of health insurance, it’s not clear if health insurance companies will be buying that information and then deciding whether to offer you a health insurance product or not.”

If asked for a ZIP code, Haver recommends declining politely. Some others say, give a fake ZIP, but Haver says he’s not a fan of being devious.